What is the origin of Bekenstein-Hawking [1]}, [2]} entropy \(S_{\pm }=\frac{A_{\pm }}{4}\) 
at the microscopic level?
This is a genuine topic of research in quantum gravity. Here \(A_{\pm }\)  is the area of the
BH EH and CH. There has been a significant success in four and
five dimensions for
supersymmetric Bogomol'ni-Prasad-Sommerfield (BPS) class of BHs [3]}. Where
it is demonstrated that the microscopic degrees of freedom should be explained
in terms of 2D conformal-field-theory (CFT).
If there exists BH entropy (\(S_{+}\) ) for outer horizon then it is also equally
true that there exists BH entropy (\(S_{-}\) ) for inner horizon. For maximally
spinning spacetime, the microscopic entropy could be found in  [4]}.
However, the detailed structure of the microscopic entropy of non-extremal BH still
to be unknown to us. Some hopeful progress have been reported in  [5]}.
